A person under arrest must be brought before a magistrate without "unnecessary delay." At this time, the arresting officer must swear to a complaint, establishing probable cause, if the arrest was without a warrant. The defendant will then be advised of the nature of the complaint, of the right to retain counsel or to be appointed counsel, and of the general circumstances under which pretrial release may be obtained.

The Eighth Amendment prohibits "excessive bail." The Supreme Court has recently held that the amendment doesn't absolutely require pretrial release, only that if bail is appropriate, it be set no higher than necessary to assure the defendant's presence at trial. Bail may be denied and the defendant held in custody if the judge decides release would pose a serious threat to the safety of the community or to a particular individual, or that no amount of bail or particular conditions of release would prevent the defendant from fleeing.

At the initial appearance, a defendant who hasn't yet been indicted also is informed of his right to a preliminary examination. This is a formal, adversary hearing, before a judge, at which the defendant may be represented by counsel and can cross-examine witnesses. The purpose of the hearing is to determine whether there is probable cause to hold the defendant for further proceedings, not to establish guilt or innocence. Because of its limited purpose, hearsay and even illegally obtained evidence can be admissible. Motions to suppress must be made to the trial court.

If the magistrate determines at the preliminary examination that there isn't probable cause, the complaint will be dismissed and the defendant released. The government may, however, institute a subsequent prosecution for the same offense, presumably with better evidence. Preliminary examinations often are used by defense counsel as an opportunity to get free "discovery" of the details of the prosecution's case. For this reason, some prosecutors prefer to obtain a grand jury indictment. An indictment is held to satisfy the probable cause requirements of the Fifth Amendment, and precludes the need for the preliminary examination.